Australia's 4x100m freestyle relay team claimed Rio Olympic gold on Saturday in world-record time.
The team of Emma McKeon, Brittany Elmslie, Bronte Campbell and sister Cate Campbell ensured that Australia defended its Olympic title. They clocked three minutes 30.65 seconds, bettering Australia's previous world-best mark of 3:30.98 set at the 2014 Glasgow Commonwealth Games.
McKeon had earlier been the second-fastest qualifier for Sunday night's 100m butterfly final. She finished second in her 'fly semifinal and backed that up with a solid leg that left the Australians in second place as Simone Manuel and Abbey Weitzeil put the United States ahead at the midway point.
But Bronte and Cate Campbell were simply too strong on the final two legs. Bronte swam the third leg and took a lead in her last 50m before world record-holder Cate iced the gold medal win with a sterling last leg as she pulled away from U.S. anchor Katie Ledecky.
"It was amazing. I jumped out of the water. I barely had any breath left, just enough to cheer Cate home," Bronte Campbell told Channel Seven. "Not that she needed it. She was just fantastic."
"It's an absolute honour to swim against Katie Ledecky, but to be honest, I knew she had no chance," Cate Campbell said.
The U.S. had to settle for silver in an American-record time of 3:31.89. That extended a gold medal drought in the event that goes back to the 1996 Atlanta Games.
"It made it so much more fun to have a relay as the first event," Ledecky said. "We couldn't be happier with our silver medal and American record."
Canada took the bronze in 3:32.89.
The relay victory came on the heels of Mack Horton's victory in the men's 400 freestyle and capped an impressive first night for an Australian team that captured only one swimming gold medal in London four years ago.
Horton's victory came at the expense of Sun Yang, the defending Olympic champion. The Aussie grabbed the lead for good on the next-to-last lap and held off the hard-charging Chinese star, who won both the 400 and 1,500 free at the 2012 London Games.
What happened after the race was even more dramatic. The bad blood between the two was on display for all to see as Horton celebrated without even acknowledging the runner-up. Sun made a move as though he wanted to congratulate Horton, but the winner looked the other way. They got out of the pool together but acted as if the other didn't exist.
After the morning prelims, Horton described Sun as "a drug cheat" when discussing an incident between the two in the practice pool. The Aussie didn't back down with a gold around his neck, referring to a three-month suspension that Sun served in 2014 for using a banned stimulant.
"I used the words drug cheat because he tested positive," Horton said. "I just have a problem with athletes who have tested positive and are still competing."
Horton's winning time was 3:41.55, just 13-hundredths of a second ahead of Sun. Italy's Gabriele Detti rallied past American Conor Dwyer, the top qualifier in the prelims, for the bronze.
"I am clean," Sun insisted. "I've done whatever it takes to prove I'm a clean athlete."
Horton and Sun finally gave each other a begrudging handshake on the medal stand, but that was about it.
"It's an absolute honour to swim against Katie Ledecky, but to be honest I knew she had no chance." Cate Campbell
Horton's compatriot, Jake Packard, missed out on qualifying for the final of the 100m breaststroke by 0.04 seconds. He was ninth through Saturday night's semifinals.
Meanwhile, Thomas Fraser-Holmes (sixth) and Travis Mahoney (seventh) were well out of medal contention in the final of the men's 400m individual medley. Japan's Kosuke Hagino took gold in that event by holding off Chase Kalisz of the United States and becoming the first non-American since 1992 to win the grueling event.
Hagino and Japanese teammate Daiya Seto raced away from the field on the butterfly and backstroke legs before Kalisz began to close the gap. The American surged past Seto on the breaststroke and set his sights on Hagino.
But the Japanese swimmer, who settled for bronze in this event at the 2012 London Games, held on to win in 4:06.05. Kalisz settled for the silver in 4:06.75, while Seto finished much farther back to get bronze.
"I thought about a lot of stuff this afternoon but decided to just stop over-thinking and go for it," Hagino said. "I knew Daiya and Chase would come for me but I just had to leave it all out there, which is what I did."
Ryan Lochte was the defending Olympic champion, but he finished third at the U.S. trials and didn't qualify. Michael Phelps was the champion in 2004 and 2008, but he has dropped the 400 IM from his program. Tom Dolan was back-to-back champion in 1996 and 2000. Tamas Darnyi of Hungary won gold at the 1992 Barcelona Games.
After repeated frustration on the sport's biggest stage, Katinka Hosszu of Hungary crushed the world record in the women's 400m individual medley Saturday night to make the first Olympic medal of her career gold. She defiantly pumped her chest before breaking into a huge smile.
"I've been chasing that world record for quite some time. It's been over seven years I've been thinking about it," she said. "I knew I could go faster, but I didn't think I can go this much faster."
Hosszu, known as the "Iron Lady" for her grueling schedule, led all the way and touched in 4 minutes, 26.36 seconds, easily eclipsing the record of 4:28.43 held by China's Ye Shiwen.
Hosszu had time to turn toward the scoreboard and savor her triumph before Maya DiRado of the United States touched in 4:31.15 to take the silver medal. Mireia Belmonte Garcia of Spain claimed the bronze in 4:32.39.
"I honestly didn't even see her, she was so far ahead," DiRado said of Hosszu.
This triumph was especially sweet for Hosszu, who had captured nine medals -- including five golds -- at the world championships but never won an Olympic medal. She defiantly pumped her chest before breaking into a huge smile.
Elizabeth Beisel of the U.S., the silver medalist at the 2012 London Games, finished sixth.
Information from AAP and The Associated Press was used in this report.